Method and apparatus for combined exhaust and compression release engine braking

ABSTRACT

A controller of an internal combustion engine receives a request for engine braking and, in response thereto, activates an exhaust braking subsystem. Additionally, after passage of a period of time, the controller further activates a compression release braking subsystem. The period of time is preferably selected to permit development of increased back pressure in an exhaust system of the internal combustion engine prior to activation of the compression release braking subsystem. Additionally, following activation of the exhaust braking subsystem, the controller may whether the exhaust braking subsystem has failed and, if so, cause the compression release braking subsystem to operate in a reduced braking power mode, for example at less than full braking power potentially down to and including no braking power.

CROSS-REFERENCE TO RELATED APPLICATION

The instant application claims the benefit of Provisional U.S. PatentApplication Ser. No. 62/213,002 entitled “System and Method forControlling Backpressure and System Loading” and filed Sep. 1, 2015, theteachings of which are incorporated herein by this reference.

FIELD

The instant disclosure relates generally to engine braking and, moreparticularly, to a method and apparatus for combined exhaust andcompression release engine braking.

BACKGROUND

Engine braking systems have been known and used for decades inconjunction with internal combustion engines, particularly dieselengines. Such systems include compression release brakes and exhaustbrakes. These braking systems may be used alone or in combination withthe other.

In simple terms, a compression release brake takes the load off thestandard service brake by turning the internal combustion engine into apower-absorbing air compressor using a compression-release mechanism.When a compression release type brake is activated, the exhaust valvesof one or more unfueled cylinders are opened near the top of thecompression stroke. This releases the highly compressed air through theexhaust system with little energy returned to the piston. As the cyclerepeats, the energy of the vehicle's forward motion (as transmittedthrough the vehicle's drive train to the engine) is dissipated, causingthe vehicle to slow down.

In contrast, an exhaust brake uses exhaust back pressure within theengine to significantly increase braking power by restricting the flowof exhaust gases and increasing back pressure inside the engine. As usedherein, engine exhaust back pressure is that pressure produced by theengine to overcome the hydraulic resistance of the engine's exhaustsystem in order to discharge the gases into the atmosphere. Theincreased backpressure in the engine creates resistance against thepistons, slowing the crankshafts rotation and helping to control thevehicle speed.

As known in the art, compression release and exhaust engine brakes canbe used together to achieve substantial levels of braking power.Unfortunately, one of the disadvantages in a combination compressionrelease and exhaust brake is high system loading seen by the overhead orvalve train, i.e., those components that normally transmit valveactuation motions to the engine valves, such as cams, rocker arms, camfollowers (roller or flat), etc. particularly during a transient event.An example of this is illustrated in FIG. 1.

In particular, FIG. 1 illustrates a control signal 104 used to controloperation of an exhaust engine brake subsystem, another control signal106 used to control operation of a compression release engine brakesubsystem and a trace 102 illustrating the force applied to valve traincomponents resulting from cylinder pressure as a function of time,measured in seconds. As known in the art, each peak illustrated in thetrace 102 represents the peak forces applied to the valve train througheach piston cycle of a given engine cylinder. When the control signals104, 106 transition, in this example, from low to high to activate boththe compression release and exhaust engine brake subsystems atapproximately the same time, this results in a typical compressionrelease transient peak 108, followed by an extended period of unusuallyhigh peaks 110 prior to normal, steady state operation 112. As a resultof the period of excessively high loads 110, damage may be inflicted onthe valve train or overhead.

Techniques that overcome these problems would represent a welcomeadvance in the art.

SUMMARY

The instant disclosure describes methods and apparatuses for combinedexhaust and compression release engine braking that substantiallyovercomes the above-noted deficiencies of prior art systems. In a firstembodiment, a controller used in conjunction with an internal combustionengine receives a request for engine braking and, in response thereto,activates an exhaust braking subsystem of the internal combustionengine. Additionally, after passage of a period of time, the controllerfurther activates a compression release braking subsystem of theinternal combustion engine. The period of time is preferably selected topermit development of increased back pressure in an exhaust system ofthe internal combustion engine.

In a second embodiment that may be performed in addition to, orseparately from, the first embodiment, the controller activates theexhaust braking subsystem in response to the request for engine braking.In this embodiment, the controller further determines, followingactivation of the exhaust braking subsystem, whether the exhaust brakingsubsystem has failed and, if so, the controller causes the compressionrelease braking subsystem to operate in a reduced braking power mode,for example at less than full braking power potentially down to andincluding no breaking power. The reduced braking power mode may beachieved by the controller activating only a portion of the compressionrelease braking subsystem. Additionally, the controller may cause analteration in configuration of the exhaust system in an effort toincrease backpressure in the exhaust system. In order to determine ifthe exhaust braking subsystem has failed, the controller may determinethat back pressure in the exhaust system is higher than a threshold. Inone implementation, the determination that the back pressure hasexceeded the threshold is further based on a determination that boostpressure in an intake system of the internal combustion engine is higherthan a threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

The features described in this disclosure are set forth withparticularity in the appended claims. These features and attendantadvantages will become apparent from consideration of the followingdetailed description, taken in conjunction with the accompanyingdrawings. One or more embodiments are now described, by way of exampleonly, with reference to the accompanying drawings wherein like referencenumerals represent like elements and in which:

FIG. 1 is time plot of control signals and forces applied to valve traincomponents in systems comprising exhaust and compression release enginebraking subsystems and in accordance with prior art techniques;

FIG. 2 is a schematic, cross-sectional view of an engine cylinder andvalve actuation systems in accordance with the instant disclosure;

FIG. 3 is a schematic illustration of an internal combustion engine inaccordance with the instant disclosure;

FIG. 4 is a flowchart illustrating processing in accordance with theinstant disclosure; and

FIG. 5 is a is time plot of control signals and forces applied to valvetrain components in systems comprising exhaust and compression releaseengine braking subsystems and in accordance with the instant disclosure.

DETAILED DESCRIPTION OF THE PRESENT EMBODIMENTS

FIG. 2 schematic, cross-sectional view of an engine cylinder and valveactuation systems in accordance with the instant disclosure. As shown,the engine cylinder 202 has disposed therein a piston 204 thatreciprocates upward and downward repeatedly during both positive powergeneration (i.e., combustion of fuel to drive the piston 204) and enginebraking operation (i.e., use of the piston to achieve air compression)of the cylinder 202. At the top of the cylinder 202, there may be atleast one intake valve 206 and at least one exhaust valve 208. Theintake valve 206 and the exhaust valve 208 may be opened and closed toprovide communication with an intake gas passage 210 and an exhaust gaspassage 212, respectively. The intake valve 206 and exhaust valve 208may be opened and closed by valve actuating subsystems 214, such as, forexample, an intake valve actuating subsystem 216, a positive powerexhaust valve actuating subsystem 218, and an engine braking exhaustvalve actuating subsystem 220. The positive power exhaust valveactuating subsystem 218 and the engine braking exhaust valve actuatingsubsystem 220 may be integrated into a single system in some embodimentsor separate in others.

The valve actuating subsystems 214 may include any number of mechanical,hydraulic, hydro-mechanical, electromagnetic, or other type of valvetrain element. For example, as known in the art, the exhaust valveactuating subsystems 218 and/or 220 may include one or more cams, camfollowers, rocker arms, valve bridges, push tubes, etc. used to transfervalve actuation motion to the exhaust valves 208. Additionally, one ormore lost motion components may be included in any of the valveactuation subsystems 214 whereby some or all of the valve actuationmotions typically conveyed by the valve actuation subsystems 214 areprevented from reaching the valves 206, 208, i.e., they are “lost.”

The valve actuating subsystems 214 may actuate the intake valve 206 andexhaust valve 208 to produce engine valve events, such as, but notlimited to: main intake, main exhaust, compression release braking, andother auxiliary valve actuation motions. The valve actuating subsystems214 may be controlled by a controller 222 to selectively control, forexample, the amount and timing of the engine valve actuations. Thecontroller 222 may comprise any electronic, mechanical, hydraulic,electrohydraulic, or other type of control device for communicating withthe valve actuating subsystems 214 and causing some or all of thepossible intake and exhaust valve actuations to be transferred to theintake valve 206 and the exhaust valve 208. The controller 222 mayinclude a microprocessor and instrumentation linked to other enginecomponents to determine and select the appropriate operation of theengine valves based on inputs such as engine speed, vehicle speed, oiltemperature, coolant temperature, manifold (or port) temperature,manifold (or port) pressure, cylinder temperature, cylinder pressure,particulate information, other exhaust gas parameters, driver inputs(such as requests to initiate engine braking), transmission inputs,vehicle controller inputs, engine crank angle, and various other engineand vehicle parameters. In particular, and in accordance withembodiments described in further detail below, the controller mayactivate the engine braking exhaust valve actuating subsystem 220 inresponse to a request for engine braking.

As noted above, pressure developed in the cylinder 202 throughreciprocation of the piston 204 places loads on the valve actuationsubsystems 214 during opening of the engine valves 206, 208. Forexample, when the piston 204 is at or near its bottom dead centerposition, pressure within the cylinder 202 will be relatively low andthe load placed on the valve actuation subsystems 214 when openingeither valve 206, 208 will be relatively low as well. On the other hand,when the piston 204 is at or near its top dead center position, pressurewithin the cylinder 202 will be relatively high and the load placed onthe valve actuation subsystems 214 when opening either valve 206, 208will be relatively high as well. This latter scenario is particularlytrue where, unlike positive power generation operation, the exhaustvalve 208 is initially opened when the piston 204 is very close to itstop dead center position.

FIG. 2 further illustrates the concept of back pressure 213 where, inthis context, the resistance of hydraulic fluid flow of the exhaustsystem manifests itself as a force applied to the exhaust valve 208 inopposition to the pressure induced in the cylinder 202. As known in theart, activation of an exhaust braking system results in increased backpressure within the exhaust system. However, such increased backpressure may take a period of time to develop. Given this, and withreference once again to FIG. 1, the period of excessively high loads110, referred to herein as a mass flow inertia pulse (MFIP), resultsfrom the sudden application of the cylinder pressures developed by thecompression release braking subsystem prior to development of increasedback pressure that would otherwise oppose the load applied to the valvetrain 218, 220.

Referring now to FIG. 3, an internal combustion engine 300 is shownoperatively connected to an exhaust system 330. The internal combustionengine 300 comprises a plurality of cylinders 302, an intake manifold304 and an exhaust manifold 306. As will be appreciated by those ofskill in the art, the number and configuration of the cylinders 302 aswell as the configuration of the intake and exhaust manifolds 304, 306may differ from the illustrated example as a matter of design choice.FIG. 3 also schematically illustrates a compression release brakingsubsystem 220 for actuating one or more exhaust valves, as known in theart. In turn, the exhaust system 330 comprises, in addition to the usualpiping, an exhaust braking subsystem 332 and, in the illustratedembodiment, a turbocharger 334. As known in the art, the turbocharger334 may comprise a turbine 336 operatively connected to a compressor 338in which exhaust gases (illustrated by the black arrows) output by theexhaust manifold 306 rotate the turbine 336 that, in turn, operates thecompressor 338. The exhaust braking subsystem 332 may comprise any of anumber of commercially available exhaust brakes.

As further shown in FIG. 3, various components may form an intake systemthat provide air to the intake manifold 304. In the illustrated example,an inlet pipe 308 provides ambient air to the compressor 338 that, inturn, provides pressurized air through a compressor outlet pipe 310 to acharge air cooler 312 that cools down the pressurized air. The output ofthe charge air cooler 312 routes the cooled, compressed air to an intakemanifold inlet 314. As known in the art, the level of compression (orboost pressure) provided by the compressor 338 depends upon the pressureof the exhaust gases escaping through the exhaust system 330.

As further shown in FIG. 3, a controller 222 is provided and operativelyconnected to the compression release braking subsystem 220 and theexhaust braking subsystem 332. In this manner, the controller 222controls operation of both the compression release braking subsystem 220and the exhaust braking subsystem 332. In the illustrated embodiment,the controller 222 comprises a processor or processing device 342coupled a storage component or memory 344. The memory 204, in turn,comprises stored executable instructions and data. In an embodiment, theprocessor 342 may comprise one or more of a microprocessor,microcontroller, digital signal processor, co-processor or the like orcombinations thereof capable of executing the stored instructions andoperating upon the stored data. Likewise, the memory 204 may compriseone or more devices such as volatile or nonvolatile memory including butnot limited to random access memory (RAM) or read only memory (ROM).Processor and storage arrangements of the types illustrated in FIG. 2are well known to those having ordinary skill in the art. In oneembodiment, the processing techniques described herein are implementedas a combination of executable instructions and data within the memory344 executed/operated upon by the processor 342.

While the controller 222 has been described as one form for implementingthe techniques described herein, those having ordinary skill in the artwill appreciate that other, functionally equivalent techniques may beemployed. For example, as known in the art, some or all of thefunctionality implemented via executable instructions may also beimplemented using firmware and/or hardware devices such as applicationspecific integrated circuits (ASICs), programmable logic arrays, statemachines, etc. Furthermore, other implementations of the controller 222may include a greater or lesser number of components than thoseillustrated. Once again, those of ordinary skill in the art willappreciate the wide number of variations that may be used is thismanner. Further still, although a single controller 222 is illustratedin FIG. 3, it is understood that a combination of such processingdevices may be configured to operate in conjunction with, orindependently of, each other to implement the teachings of the instantdisclosure.

Referring now to FIG. 4, processing in accordance with the instantdisclosure is illustrated. In particular, the processing illustrated inFIG. 4 may be implemented by the controller 222 as described above.Beginning at block 402, the controller receives a request for enginebraking. As noted above, such a request may be provided in the form of auser input such as through activation of a switch or otheruser-selectable mechanism as known in the art. Regardless, responsive tothe received request, processing continues at block 404 where thecontroller 404 activates the exhaust braking subsystem. As known in theart, activation of a braking subsystem (whether exhaust, compressionrelease or another type) may be effected through control, for example,of a solenoid that, in turn, controls the flow of hydraulic fluid to alost motion system or actuator that initiates the engine brakingoperation. An example of a signal 504 used for this purpose isillustrated in FIG. 5 where the transition of the signal 504 from a lowvoltage to a high voltage corresponds to activation of the exhaustbraking system. Those having skill in the art will appreciate that theparticular form of control signals illustrated herein are not by way oflimitation and, in practice, other forms (e.g., high to lowtransitioning) may be equally employed.

Thereafter, at block 408, a determination is made if a period of timehas been completed subsequent to the activation of the exhaust brakingsystem. That is, substantially simultaneous with the activation of theexhaust braking subsystem, the controller initiates a timer measuringthe period of time in accordance with well-known techniques and thencontinually checks 408 whether the timer has expired (in this example).In an embodiment, the period of time is sufficient in length to permitactivation of the exhaust break to develop increased back pressure inthe exhaust system such that the loads placed upon the exhaust valvetrain may the cylinder pressure may be more effectively opposed, therebyminimizing or eliminating any period of high loads 110, as describedabove. In practice, the desired period of time will be function ofengine speed, exhaust gas flow and volume of the exhaust system and willtherefore necessarily vary depending upon the specific implementationand operation of the engine and exhaust system. For example, testing hasrevealed that in some commonly available engine and exhaust systems, theperiod of time should be at least one second.

Regardless of the specific period of time employed, once the period oftime has passed, processing continues at block 410 where the compressionrelease braking subsystem is activated. This is once again shown in FIG.5 where, after the completion of the period of time 510, a controlsignal 506 for the compression release braking subsystem transitionsfrom a low voltage to a high voltage. Consequently, as further shown inFIG. 5, the forces applied to the valve train are increased beginningwith a typical compression release transient 508. However, unlike thesystem illustrated in FIG. 1, there is no or very little MFIP-inducedperiod of excessively high loads 110. Once again, the backpressuredeveloped during the period of time 510 during which only the exhaustbraking subsystem is activated significantly counteracts the otherwisehigh loads 110 that would result.

As known in the art, failure of an exhaust braking subsystem can havesignificant deleterious effects on an engine. If the exhaust brakingsubsystem fails in a way in which the restriction in the exhaust systemis maintained even after the exhaust braking subsystem has beendeactivated, there will be significant increase in back pressure duringpositive power generation, which can decrease positive power generationand, in turbocharger-equipped systems, decrease the boost pressure. Onthe other hand, if the exhaust braking subsystem fails in a way in whichthe restriction in the exhaust system is not provided when the exhaustbraking subsystem has been activated, there will be significant decreasein back pressure during engine braking that, as described above, canresult in damage to valve train components.

In order to avoid the potentially damaging effects of failure of theexhaust braking subsystem during engine braking in a combinedexhaust/compression release engine braking system, the otherwiseoptional processing (illustrated with dotted lines) shown in FIG. 5 mayalso be performed. It is note that, which the optional processing inFIG. 5 is shown in conjunction with the described method for minimizingor eliminating the high valve train loads 110 described above, this isnot a requirement of the instant disclosure. That is, the otherwiseoptional processing in FIG. 5 (particularly blocks 406 and 414) inconjunction with the processing of blocks 402 and 404 may be implantedseparately, e.g., in a system in which only an exhaust braking subsystemis provided.

Regardless, in this additional embodiment, subsequent to activation ofthe exhaust braking subsystem, it is determined at block 406 whetherthere has been a failure of the exhaust braking subsystem. In practice,this may be achieved in several ways. In particular, where the exhaustbraking subsystem fails to provide the necessary restriction on theexhaust system, this failure can be detected when it is determined thatback pressure in the exhaust system is below a threshold. For example,and with reference to FIG. 3, this could be determined throughmeasurement of pressure within the exhaust manifold or that portion ofthe exhaust system between the turbocharger 334 and the exhaust brakingsubsystem 332. In yet another embodiment, it is appreciated thatpresence of the restriction in the exhaust system through normaloperation of the exhaust braking subsystem will serve to reduce theboost pressure at the outlet of the compressor 338 (or as measured, forexample, in the intake manifold 304 or either before or after the chargeair cooler 312). Consequently, a measurement of a sustained or evenincreased boost level after activation of the exhaust braking subsystemis indicative of a failure of the exhaust braking subsystem to providethe necessary restriction.

Regardless of the manner in which it is determined, if no failure of theexhaust braking subsystem is determined at block 406, processingcontinues as described above at block 408. However, if a failure isdetected at block 406, processing continues at block 414 where, ratherthan activating the compression release braking subsystem in the usualmanner at block 410, the compression release braking system is operatedin a reduced braking power mode. As used herein, a reduced braking powermode is characterized by less than the full braking power that couldotherwise be provided by the compression release braking subsystem downto, and including, no braking power at all. For example, to achieve areduced braking power mode, the controller could operate the compressionrelease braking subsystem in such a manner that only a portion of thecompression release braking subsystem is operated. Thus, in oneembodiment, not all of the cylinders may be operated in accordance withcompression release engine braking techniques. In another embodiment,where possible, the timing of the opening of the exhaust valves duringcompression release braking could be modified such that they are notopened at or close to periods of peak cylinder pressure, therebydecrease the loads that would otherwise be placed upon the valve trains.

In yet another embodiment, where possible, the controller may alsoconfigure one or more components of the exhaust system (other than theexhaust braking subcomponent) to increase the back pressure in theexhaust system. For example, and with reference to FIG. 3, if theturbocharger 334 is a so-called variable geometry turbocharger (VGT) asknown in the art, the configuration of the turbocharger (for example,the aspect ratio of the turbine blade in the turbine 336) may beadjusted to increase the back pressure of the exhaust system 330.

Further still, even when no failure is detected at block 406 andcompression release braking is activated as set forth in block 410, itmay be desirable to continue checking for failure of the exhaust brakingsubsystem as illustrated by block 412. In the case that such a failureis detected even after activation of the compression release brakingsubsystem, processing may continue at block 414 where a reduced brakingpower mode of operation is employed, as described above.

While particular preferred embodiments have been shown and described,those skilled in the art will appreciate that changes and modificationsmay be made without departing from the instant teachings. It istherefore contemplated that any and all modifications, variations orequivalents of the above-described teachings fall within the scope ofthe basic underlying principles disclosed above and claimed herein.

What is claimed is:
 1. In a controller for use with an internalcombustion engine operatively connected to an exhaust system, whereinthe internal combustion engine comprises a compression-release brakingsubsystem and the exhaust system comprises an exhaust braking subsystem,and wherein the controller is in communication with thecompression-release braking subsystem and the exhaust braking subsystem,a method for performing engine braking comprising: receiving, by thecontroller, a request for engine braking; responsive to the request forengine braking, activating, by the controller, the exhaust brakingsubsystem; and responsive to the request for engine braking and aftercompletion of a period of time following activation of the exhaustbraking subsystem, activating, by the controller, thecompression-release braking subsystem.
 2. The method of claim 1, whereinthe period of time is sufficient to develop increased backpressure inthe exhaust system.
 3. The method of claim 2, wherein the period of timeis at least one second.
 4. The method of claim 1, further comprising:determining, by the controller, that the exhaust braking subsystem hasfailed; and subsequent to determining that the exhaust braking subsystemhas failed, operating, by the controller, the compression-releasebraking subsystem in a reduced braking power mode.
 5. The method ofclaim 4, wherein determining that the exhaust braking subsystem hasfailed further comprises: determining, by the controller, thatbackpressure in the exhaust system is lower than a threshold.
 6. Themethod of claim 4, wherein determining that the exhaust brakingsubsystem has failed further comprises: determining, by the controller,that boost pressure in an intake subsystem of the internal combustionengine is higher than a threshold.
 7. The method of claim 4, whereinoperating the compression-release braking subsystem in the reducedbraking power mode further comprises operating the compression-releasebraking subsystem at less than full braking power up to and including nobraking power.
 8. The method of claim 4, wherein operating thecompression-release braking subsystem in the reduced braking power modefurther comprises activating, by the controller, only a portion of thecompression-release braking subsystem.
 9. The method of claim 4, furthercomprising: altering, by the controller, configuration of the exhaustsystem to increase backpressure in the exhaust system.
 10. A controllerfor use with an internal combustion engine operatively connected to anexhaust system, wherein the internal combustion engine comprises acompression-release braking subsystem and the exhaust system comprisesan exhaust braking subsystem, and wherein the controller is incommunication with the compression-release braking subsystem and theexhaust braking subsystem, the controller comprising: at least oneprocessing device; and memory having stored thereon executableinstructions that, when executed by the at least one processing devicecause the at least one processing device to: receive a request forengine braking; responsive to the request for engine braking, activatethe exhaust braking subsystem; and responsive to the request for enginebraking and after completion of a period of time following activation ofthe exhaust braking subsystem, activate the compression-release brakingsubsystem.
 11. The controller of claim 10, wherein the period of time issufficient to develop increased backpressure in an exhaust systemoperatively connected to the internal combustion engine.
 12. Thecontroller of claim 10, wherein the period of time is at least onesecond.
 13. The controller of claim 10, the memory further comprisingexecutable instructions that, when executed by the at least oneprocessor, cause the at least one processor to: determine that theexhaust braking subsystem has failed; and subsequent to determining thatthe exhaust braking subsystem has failed, operate thecompression-release braking subsystem in a reduced braking power mode.14. The controller of claim 13, wherein those executable instructionsthat cause the at least one processor to determine that the exhaustbraking subsystem has failed are further operative to cause the at leastone processor to: determine that backpressure in the exhaust system islower than a threshold.
 15. The controller of claim 13, wherein thoseexecutable instructions that cause the at least one processor todetermine that the exhaust braking subsystem has failed are furtheroperative to cause the at least one processor to: determine that boostpressure in an intake subsystem of the internal combustion engine ishigher than a threshold.
 16. The controller of claim 13, wherein thoseexecutable instructions that cause the at least one processor to operatethe compression-release braking subsystem in the reduced braking powermode further are further operative to cause the at least one processorto operate the compression-release braking subsystem at less than fullbraking power up to and including no braking power.
 17. The controllerof claim 13, wherein those executable instructions that cause the atleast one processor to operate the compression-release braking subsystemin the reduced braking power mode further are further operative to causethe at least one processor to activate only a portion of thecompression-release braking subsystem.
 18. The controller of claim 13,the memory further comprising executable instructions that, whenexecuted by the at least one processor, cause the at least one processorto: alter configuration of the exhaust system to increase backpressurein the exhaust system.
 19. An internal combustion engine comprising thecontroller of claim 10
 20. In a controller for use with an internalcombustion engine operatively connected to an exhaust system, whereinthe internal combustion engine comprises a compression-release brakingsubsystem and the exhaust system comprises an exhaust braking subsystem,and wherein the controller is in communication with thecompression-release braking subsystem and the exhaust braking subsystem,a method for performing engine braking comprising: receiving, by thecontroller, a request for engine braking; responsive to the request forengine braking, activating, by the controller, the exhaust brakingsubsystem; determining, by the controller, that the exhaust brakingsubsystem has failed; and subsequent to determining that the exhaustbraking subsystem has failed, operating, by the controller, thecompression-release braking subsystem in a reduced braking power mode.21. The method of claim 20, wherein determining that the exhaust brakingsubsystem has failed further comprises: determining, by the controller,that backpressure in the exhaust system is lower than a backpressurethreshold.
 22. The method of claim 20, wherein determining that theexhaust braking subsystem has failed further comprises: determining, bythe controller, that boost pressure in an intake subsystem of theinternal combustion engine is higher than a threshold.
 23. The method ofclaim 20, wherein operating the compression-release braking subsystem inthe reduced braking power mode further comprises operating thecompression-release braking subsystem at less than full braking power upto and including no braking power.
 24. The method of claim 20, whereinoperating the compression-release braking subsystem in the reducedbraking power mode further comprises activating, by the controller, onlya portion of the compression-release braking subsystem.
 25. The methodof claim 20, further comprising: altering, by the controller,configuration of the exhaust system to increase backpressure in theexhaust system.
 26. A controller for use with an internal combustionengine operatively connected to an exhaust system, wherein the internalcombustion engine comprises a compression-release braking subsystem andthe exhaust system comprises an exhaust braking subsystem, and whereinthe controller is in communication with the compression-release brakingsubsystem and the exhaust braking subsystem, the controller comprising:at least one processing device; and memory having stored thereonexecutable instructions that, when executed by the at least oneprocessing device cause the at least one processing device to: receive arequest for engine braking; responsive to the request for enginebraking, activate the exhaust braking subsystem; determine that theexhaust braking subsystem has failed; and subsequent to determining thatthe exhaust braking subsystem has failed, operate thecompression-release braking subsystem in a reduced braking power mode.27. The controller of claim 26, wherein those executable instructionsthat cause the at least one processor to determine that the exhaustbraking subsystem has failed are further operative to cause the at leastone processor to: determine that backpressure in the exhaust system islower than a backpressure threshold.
 28. The controller of claim 26,wherein those executable instructions that cause the at least oneprocessor to determine that the exhaust braking subsystem has failed arefurther operative to cause the at least one processor to: determine thatboost pressure in an intake subsystem of the internal combustion engineis higher than a threshold.
 29. The controller of claim 26, whereinthose executable instructions that cause the at least one processor tooperate the compression-release braking subsystem in the reduced brakingpower mode further are further operative to cause the at least oneprocessor to operate the compression-release braking subsystem at lessthan full braking power up to and including no braking power.
 30. Thecontroller of claim 26, wherein those executable instructions that causethe at least one processor to operate the compression-release brakingsubsystem in the reduced braking power mode further are furtheroperative to cause the at least one processor to activate only a portionof the compression-release braking subsystem.
 31. The controller ofclaim 26, the memory further comprising executable instructions that,when executed by the at least one processor, cause the at least oneprocessor to: alter configuration of the exhaust system to increasebackpressure in the exhaust system.
 32. An internal combustion enginecomprising the controller of claim 26.